Transparent electrically-conductive films, obtained by forming a transparent electrically-conductive thin film of tin oxide, indium oxide, indium-tin complex oxide, zinc oxide, etc., on a transparent polymeric film, are widely used for electro-optical devices such as a liquid crystal display element, an organic EL display element or a tranparent electrode of a touch panel.
As a transparent polymeric film to be used as a base material, for example, there is used polyethylene terephthalate or polycarbonate, as disclosed in JP-A-06-251632. However, since these films are insufficient in heat resistance, it is required to keep a process temperature for the production of a transparent electrically-conductive thin film low. When the temperature is kept low, a defect is that the crystallinity of the transparent electrically-conductive film decreases so that the surface resistivity of the film increases.
Further, as a constituent material for a plastic substrate having high heat resistance, there is partially used a polyarylate resin or a polyether sulfone resin, as disclosed in, for example, JP-A-06-044826. However, when a transparent electrically-conductive thin film is produced at a high temperature of 200° C. or higher, or when a thin film transistor (TFT) is formed in addition to the transparent electrically-conductive thin film, even the above-level heat-resistance is insufficient.
For example, an active matrix display device using a thin film transistor substrate obtained by forming a thin film transistor as a pixel-switching element on a glass substrate is adopted in recent years, since it can respond to the needs of upsizing, high-definition and high-intensity of a device. If a substrate of a plastic film can be adopted in place of a glass substrate, there can be obtained a thin, light and almost unbreakable display device. As a substrate for a passive matrix display device that does not use a thin film transistor, there have been already adopted transparent plastic films such as polyethylene terephthalate, polycarbonate or polyether sulfone, as disclosed in JP-A-9-169074 or JP-A-2001-52530.
However, concerning a general process of producing a thin film transistor, a temperature for forming a polycrystalline silicon film is a high temperature of at least 400° C. even in a low-temperature process and a temperature for forming an amorphous silicon film is also a high temperature of at least 250° C. In this case, the above conventionally-used transparent plastic films are insufficient in heat resistance. It is very difficult to obtain a thin film transistor substrate by forming a thin film transistor on a substrate of one of these films.
Further, JP-A-9-116158 discloses a method in which a thermal diffusion means is provided for preventing a temperature at the time of forming a silicon film from increasing. JP-A-10-270711 discloses a method in which a film-forming temperature is decreased by adopting a special method in a film-forming operation. According to these methods, it is possible to form a thin film transistor on a conventional plastic film substrate. However, each method uses a special technique so that a defect is that a high cost is required.
On the other hand, a polyimide resin is known as a resin excellent in heat resistance or dimensional stability. A wholly aromatic polyimide resin obtainable by a polycondensation reaction of an aromatic tetracarboxylic acid dianhydride and an aromatic diamine can be used at a high temperature of 400° C. or higher and it has excellent properties of small thermal expansion coefficient and high dimensional stability. Therefore, it has been used in various fields including an aerospace industry and an electronic industry as a raw material for a film to be used at high temperatures, a wire-coating, an adhesive or a coating composition. However, since such a wholly aromatic polyimide resin has a color of from light yellow to reddish brown, it is unsuitable for a base material for the transparent electrically-conductive film. Further, it is also unsuitable for a film base material for a thin film transistor substrate used for an active matrix display device.